The use of an OFDM (orthogonal frequency-division multiplex) method, an OFDMA (orthogonal frequency-division multiplex access) method or the like as the latest technique of a radio broadband mobile communication method is studied. The OFDM method divides transmitting information using a plurality of sub carriers of different center frequencies, modulates it for each sub carrier and transmits it in parallel. Thus, symbol transfer speed per carrier can be made slower than that in the case of serial transfer by dividing information and the influence of fading can be reduced.
The OFDMA method realizes multi-access by dividing the sub carriers of the OFDM method into several groups, extracting sub carriers from each group and using each of the extracted sub carriers as a group of sub carriers for each user. Thus, the high use efficiency of frequency can be obtained compared with a conventional FDMA method.
FIG. 1 is a configuration block diagram of a conventional example of a radio base station device using, for example, such an OFDM method. This radio base station device divides a radio communication area, that is, an area where terminals accommodated in one base station into a plurality of sectors and conducts radio communications with its terminals.
In FIG. 1, a radio base station device 100 can accommodate a frequency band of 10 MHz for each of three sectors and includes three process units of a sector 1 process unit 1011 through a sector 3 process unit 1013 for each of the sectors. Each process unit, for example, the sector 1 process unit 1011. includes two basic process blocks, such as basic process blocks 1021 and 1022 of 5 MHz per carrier. In a down process, that is, at the time of transmitting to the terminal side, this basic process block performs a baseband process, such as encoding user data from an entrance line connected to an exchange or the like and outputs its result to, for example, inverse fast Fourier transfer (IFFT) process unit. In up process, that is, at the time of receiving from the terminal side, this basic process block performs a baseband process, such as decoding data from an FFT (fast Fourier transfer) process unit or the like and outputs its result to the entrance line side.
FIG. 2 explains the problem of the conventional device illustrated in FIG. 1. In FIG. 2, many user terminals are concentrated to the sector 2 area and few user terminals exist in the sector 1 and 3 areas. As explained in FIG. 1, inside the radio base station device 100, two basic process blocks are fixedly allocated to each sector process unit. Therefore, even when the amount of hardware resources of the entire base station device is sufficient, the amount of radio hardware resources becomes insufficient for the sector 2 and there is a possibility that a non-connectable call or an incomplete call in which a call is disconnected while not completed occurs.
As a conventional art for improving the frequency use efficiency of such a radio base station device, Japanese Laid-open Patent Publication No. 2005-27189 “Control Station, Radio Communication System and Frequency Allocation Method” discloses a technique for improving the frequency band use efficiency of the entire system by regularly modifying frequency (channel) allocation in such a way as to maintain an interference level constant, without fixedly allocating a frequency to each cell in an FDMA system.
However, for example, in an OFDMA system, that is, a radio communication system capable of changing frequency bandwidth, which is the subject of the present application, the technique of this patent document is unsuitable for application.